Press brake and method of controlling bidirectional fluid pump of hydraulic cylinder of press brake

ABSTRACT

A controller for a press brake controls a servomotor to reverse the rotation of a bidirectional piston pump so as to reverse the vertical movement of a hydraulic cylinder and a ram. A command generator maintains a constant speed for a predetermined warmup time or distance after the ram movement is reversed, and afterwards changes the ram speed to a predetermined speed. A ram position, set according to a moving speed pattern, is compared to the actual position of the ram to conform the ram to the moving speed pattern. The moving speed may be limited based on at least one of a pressure and a detected by a pressure sensor.

TECHNICAL FIELD

The present invention relates to a press brake which executes a bendingprocess by vertically moving a ram using a hydraulic cylinder and, inparticular, to controlling a bidirectional fluid pump that operates thehydraulic cylinder.

BACKGROUND ART

A press brake executes a bending process on the basis of cooperationbetween a punch and a die. A ram is vertically moved using a hydrauliccylinder which is operated by a bidirectional fluid pump. A hydrauliccircuit provided in the hydraulic cylinder is shown and described withrespect to FIG. 1.

In the hydraulic circuit mentioned above, pipings 101 and 103, connectedto an upper cylinder chamber or a lower cylinder chamber of, a hydrauliccylinder (not shown), are connected to a bidirectional fluid pump 107that is rotated by a servo motor 105. Further, the pipings 101 and 103are respectively connected to an oil tank 113 via check valves 109 and111.

Accordingly, the bidirectional fluid pump 107 is rotated by the servomotor 105, a working fluid is supplied to the upper or lower cylinderchamber (not shown) through the piping 101 or the piping 103, and a ram(not shown) is thereby vertically moved. The working fluid is suppliedfrom the oil tank 113 via the check valve 109 or the check valve 111.

For the hydraulic circuit mentioned above, a command is given to theservo motor 105 to rotate the bidirectional fluid pump 107 so that theram is vertically moved according to a pattern shown in FIG. 2. That is,the ram increases speed according to a fixed acceleration rate, moves ata fixed speed after reaching a predetermined speed, and reduces speedaccording to a fixed deceleration rate.

However, in the prior art mentioned above, a negative pressure may beapplied to a check valve 109 or 111 when the check valve 109 or 111 isstill open and the bidirectional fluid pump 107 is reverse rotated tochange the moving direction of the ram. When the bidirectional fluidpump 107 is reverse rotated and a positive pressure is suddenly applied,the working fluid may flow back until the open check valve 109 or 111 isclosed. When the working fluid flows back, a response is deteriorated,and the movement of the ram is unstable as shown in FIG. 3. Furthermore,a large shock occurs at a time of reverse rotation. Moreover, it isimpossible to increase a motion gain of the ram. Accordingly,productivity is reduced.

The present invention takes the problems in the prior art intoconsideration.

Accordingly, an object of the present invention is to provide a pressbrake and a method of controlling a bidirectional fluid pump of thepress brake. According to an aspect of the present invention, a motiongain of a ram can increase so as to improve productivity by reducing ashock at a time of reverse rotation.

Another object of the present invention is to provide a press brakewhich can reduce a noise generated by a bidirectional fluid pump thatoperates a hydraulic cylinder.

DISCLOSURE OF THE INVENTION

In order to achieve the objects mentioned above, according to a firstaspect of the invention, a press brake is provided. The press brakeincludes a ram capable of moving vertically and a hydraulic cylinderthat moves the ram vertically. The press brake also includes abidirectional fluid pump that operates the hydraulic cylinder in avertical direction. The bidirectional fluid pump is connected to thehydraulic cylinder and rotates forward and backward so as to move theram upward and downward.

The press brake also includes a servo motor that rotates thebidirectional fluid pump. A control apparatus controls the servo motor.The control apparatus includes a ram moving speed pattern commandportion that presets a ram moving speed pattern which maintains a ramspeed for a warmup time or distance after reversing a rotation of thebidirectional fluid pump. Thereafter, the ram speed is changed to apredetermined speed.

A command position counter reads a ram position on the basis of the ramspeed preset by the ram moving speed pattern command portion. A ramposition detector detects the actual position of the ram. An adder adds(compares) the ram position read by the command position counter and theram position signal from the ram position detector so as to give aninstruction to position the ram at a desired position.

In the structure mentioned above, in order to switch the verticalmovement of the hydraulic cylinder and the ram, the control apparatuscontrols the servo motor to reverse the rotation of the bidirectionalfluid pump. At this time, the ram moving speed pattern command portionof the control apparatus executes the pattern command of the preset rammoving speed pattern. The moving speed of the ram is fixed for thepredetermined warmup time or distance and thereafter the moving speed ofthe ram is changed to the predetermined speed. The command positioncounter reads the ram position from the ram moving speed pattern, andthe adder adds (compares) the ram moving speed pattern position and anactual ram position detected by the ram position detector, whereby therotation of the servo motor is controlled so that the ram is positionedat a desired position.

Accordingly, it is possible to reduce a shock at a time of rising and itis possible to prevent the ram from vibrating at a time of moving.Therefore, it is possible to increase a motion gain of the ram so as toimprove productivity.

According to a second aspect of the invention, there is provided amethod of controlling a bidirectional fluid pump of a hydraulic cylinderof a press brake. The method includes reversing a bidirectional fluidpump so as to reverse a vertical movement of the ram. After reversingthe bidirectional fluid pump, the method includes setting apredetermined warmup time or a warmup distance for temporarilymaintaining a moving speed of the ram. After setting the warmup time ordistance, the method includes controlling the bidirectional fluid pumpso as to change the ram speed to a predetermined speed. A bendingprocess is executed in accordance with the hydraulic cylinder beingmoved vertically in correspondence to the rotational direction of thebidirectional fluid pump. Accordingly, the ram is moved upward anddownward.

In the structure mentioned above, in the case of reversing the rotationof the bidirectional fluid pump in order to switch the vertical movementof the hydraulic cylinder and the ram, the moving speed of the ram isfixed for the predetermined warmup time or warmup distance, andthereafter the moving speed of the ram is changed to the predeterminedspeed after the reverse rotation.

Accordingly, it is possible to reduce a shock at a time of rising and itis possible to prevent the ram from vibrating at a time of moving.Therefore, it is possible to increase a motion gain of the ram so as toimprove productivity.

According to a third aspect of the invention, there is provided a methodof controlling a bidirectional fluid pump of a hydraulic cylinder of apress brake. The method includes measuring a hydraulic force in abidirectional fluid pump and computing a change amount of the hydraulicforce. The method also includes calculating a ram moving speed withrespect to a pressure detected at a certain time or a ram moving speedwith respect to a change amount of the pressure at this time. The rammoving speed is calculated on the basis of a predetermined relationshipbetween the pressure and the ram moving speed or a predeterminedrelationship between the pressure change amount and the ram movingspeed. The calculations result in minimizing a noise at a time that thebidirectional fluid pump rotates. The method also includes determining arotational amount corresponding to the calculated speed. The servo motoris instructed to rotate the bidirectional fluid pump, using thedetermined rotational amount. The bidirectional fluid pump is operatedso as to rotate the servo motor, and the ram is moved upward anddownward by the hydraulic cylinder, thereby executing a bending process.

According to a fourth aspect of the invention, there is provided amethod of controlling a bidirectional fluid pump of a hydraulic cylinderof a press brake. The method includes measuring a hydraulic force in abidirectional fluid pump and computing a change amount of the hydraulicforce. The method also includes calculating a ram moving speed withrespect to a pressure detected at a certain time and a ram moving speedwith respect to a change amount of the pressure at this time. The rammoving speeds are calculated on the basis of a predeterminedrelationship between pressure and ram moving speed or a predeterminedrelationship between a pressure change amount and ram moving speed. Thecalculations result in minimizing a noise at a time that thebidirectional fluid pump rotates. The method also includes comparing thecalculated speeds to obtain the lowest ram moving speed. The method alsoincludes determining a rotational amount corresponding to the lowestcalculated ram moving speed. The servo motor is instructed to rotate thebidirectional fluid pump, using the determined rotational amount. Themethod also includes operating the bidirectional fluid pump so as torotate the servo motor, and moving the ram upward and downward by thehydraulic cylinder, thereby executing a bending process.

According to the structure mentioned above, the control is executed bydetecting the hydraulic force of the bidirectional fluid pump rotated bythe servo motor and calculating the change amount of the hydraulicforce. The lower ram moving speed is selected on the basis of thepredetermined relationship between pressure and the ram moving speed andthe predetermined relationship between pressure change amount and theram moving speed. The calculations result in minimizing noise at a timewhen the bidirectional fluid pump rotates. The servo motor is providedwith the rotational amount corresponding to the selected ram movingspeed.

Accordingly, it is possible to restrict the noise of the bidirectionalfluid pump.

According to a fifth aspect of the invention, a press brake is provided.The press brake includes a vertically movable ram and a hydrauliccylinder that moves the ram vertically. The press brake also includes abidirectional fluid pump that operates the hydraulic cylinder in avertical direction. The bidirectional fluid pump is connected to thehydraulic cylinder and rotates forward and backward so as to move theram upward and downward. The press brake also includes a servo motorthat rotates the bidirectional fluid pump.

A ram position detector detects a position of the ram in a verticaldirection. The press brake also includes a ram moving speed patterncommand portion that provides a moving pattern of the ram. A computingportion computes a pressure, as sensed by a pressure sensor, or apressure change amount. A ram moving speed computing portion computes aram moving speed, based on the pressure or pressure change amount. Aservo motor rotation command portion provides a rotational amount,corresponding to the ram moving speed, to the servo motor.

According to a sixth aspect of the invention, a press brake is provided.The press brake includes a vertically movable ram and a hydrauliccylinder that moves the ram upward and downward. The press brake alsoincludes a bidirectional fluid pump that operates the hydraulic cylinderin a vertical direction. The bidirectional fluid pump is connected tothe hydraulic cylinder and rotates forward and backward so as to movethe ram upward and downward. The press brake also includes a servo motorthat rotates the bidirectional fluid pump.

A ram position detector detects a position of the ram in a verticaldirection. A ram moving speed pattern command portion provides a movingpattern of the ram. An adder applies a rotation command to the servomotor rotating the bidirectional fluid pump. The adder compares a ramposition from the ram moving speed pattern command portion with anactual ram position from the ram position detector so as to correct theram position.

A pressure sensor detects a pressure of the bidirectional fluid pump. Acomputing portion computes a pressure change amount on the basis of apressure signal that indicates the pressure detected by the pressuresensor. A memory stores a relation between the ram moving speed and thepressure of the bidirectional fluid pump and a relation between the rammoving speed and the pressure change amount. A servomotor rotationalamount command portion compares the relation between the ram movingspeed and the pressure of the bidirectional fluid pump with the relationbetween the ram moving speed and the pressure change amount so as toselect one having the smaller ram moving speed. The servo motorrotational amount command portion provides the rotational amountcorresponding to the ram moving speed to the servo motor.

According to the structure mentioned above, the bending process having ahigh accuracy is executed by controlling the servo motor according tothe command pattern from the ram moving speed pattern command portion.Accordingly, the hydraulic cylinder is moved upward and downward by thebidirectional fluid pump. The process includes detecting the actual ramposition by the ram position detector and comparing the instructedposition with the actual ram position so as to control the servo motor.The hydraulic force of the bidirectional fluid pump is detected by thepressure sensor provided in the bidirectional fluid pump, and thecomputing portion calculates the change amount of the hydraulic force onthe basis of the pressure. The ram speed determining portion determinesthe lower ram moving speed and selects the ram moving speed in order toreduce the noise. The ram moving speed is selected based on thepredetermined pressure-ram moving speed relation and the pressure changeamount-ram moving speed relation which are stored in the memory. Theservo motor rotational amount command portion provides the rotationalamount corresponding to the selected ram moving speed to the servomotor.

Accordingly, it is possible to restrict the noise of the bidirectionalfluid pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a main portion of a hydraulic circuitof a press brake according to a conventional art;

FIG. 2 is a graph showing a ram moving speed pattern according to theconventional art;

FIG. 3 is a graph showing an actual moving speed of a ram when movementis directed on the basis of the ram moving speed pattern shown in FIG.2;

FIG. 4 is a front elevational view showing a whole of a press brakeaccording to the present invention;

FIG. 5 is a side elevational view as seen from a direction V in FIG. 4;

FIG. 6 is a circuit and block diagram showing a structure of a hydrauliccircuit and a control apparatus in the press brake according to thepresent invention;

FIG. 7 is a graph showing a ram moving speed pattern;

FIG. 8 is a graph showing an actual moving speed of a ram when movementis instructed on the basis of the ram moving speed pattern shown in FIG.7;

FIG. 9 is a graph showing an actual speed and a pressure of the ram withrespect to a ram speed command value in a bending process;

FIG. 10 is a graph showing a rotational amount of a servo motor in thebending process shown in FIG. 9;

FIG. 11 is a graph showing a magnitude of noise with respect to therotational amount of the servo motor shown in FIG. 10;

FIG. 12 is a block diagram showing a structure of a control apparatusthat executes a method of controlling a bidirectional fluid pump of ahydraulic cylinder according to the present invention;

FIG. 13 is a graph showing an absolute amount of pressure and a changeamount of pressure during the bending process;

FIG. 14 is a graph showing a relation between a ram speed and a pressurechange amount which should be employed when taking the noise of thebidirectional fluid pump into consideration; and

FIG. 15 is a graph showing a relation between a ram speed and anabsolute amount of pressure which should be employed when taking thenoise of the bidirectional fluid pump into consideration.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be explained below in detailwith reference to the accompanying drawings.

In FIGS. 4 and 5, there is shown a whole of a press brake 1 according tothe present invention. This press brake 1 has side plates 3L and 3Rprovided in left and right sides. The press brake 1 also has an uppertable 5U serving as a ram on front end surfaces of upper portions in theside plates 3L and 3R, so as to freely move upward and downward. Thepress brake 1 is also provided with a lower table 5L on front surfacesof lower portions in the side plates 3L and 3R.

A punch P is provided in a lower end portion of the upper table 5U via aplurality of intermediate plates 7, so as to be freely replaced.Further, a die D is provided in a die holder 9 provided in an upper endportion of the lower table 5L, so as to be freely replaced.

Incidentally, an exemplary linear scale 11 operating as a ram positiondetecting means is provided for measuring a position of height of theupper table 5U. Whether or not the bending process is finished, abending angle detection and the like are executed by determining aninterval with respect to the die D on the basis of the height of thepunch P.

Hydraulic cylinders 13L and 13R are respectively provided in the frontsurfaces of the upper portions in the left and right side plates 3L and3R. The upper table 5U is mounted to piston rods 17L and 17R which areattached to pistons 15L and 15R of the hydraulic cylinders 13L and 13R.

Next, a hydraulic circuit for the hydraulic cylinders 13L and l3R and acontrol apparatus 18 will be explained with reference to FIG. 6.Incidentally, since the left and right hydraulic cylinders 13L and 13Rare provided with the same hydraulic circuit, the hydraulic cylinder 13Rand the hydraulic circuit which are provided in the right side will beexplained as follows.

An upper cylinder chamber 19U of the hydraulic cylinder 13R, for movingthe upper table 5U that corresponds to the ram upward and downward, isconnected to a prefill valve 23 by a piping 21, and is further connectedto an oil tank 27 by a piping 25.

Further, the upper cylinder chamber 19U is connected by a piping 29 toone side of a bidirectional piston pump 31 that corresponds to abidirectional fluid pump capable of rotating in two directions. A piping33 is connected to a middle of the piping 29, and is connected to theoil tank 27 via a check valve 35 and a suction filter 37. Incidentally,the bidirectional piston pump 31 is rotated by an AC servo motor 39corresponding to a servo motor controlled by the control apparatus 18.

A piping 41 is connected to a lower cylinder chamber 19L of thehydraulic cylinder 13R, and a counter balance valve 43 and a sequenceswitch valve 45, corresponding to an electromagnetic poppet valve, areprovided in parallel. The counter balance valve 43 and the sequenceswitch valve 45 are connected to another side of the bidirectionalpiston pump 31 by a piping 47. Further, a piping 49 is connected to amiddle of the piping 47, and this piping 49 is connected to the oil tank27 via a check valve 51 and a suction filter 53.

Further, a throttle valve 55 and a high pressure preference type shuttlevalve 57 are provided between the piping 41 and the piping 29. A piping59 is connected to a discharge side of the high pressure preference typeshuttle valve 57. A relief valve 61 is provided in the piping 59, and apiping 63 is connected to the oil tank 27.

The control apparatus 18 that controls the AC servo motor 39 has a rammoving speed pattern command portion 65 that provides a moving speedpattern of the upper table 5U that corresponds to the ram. In this rammoving speed pattern command portion 65, a command is given so as toreverse a vertical movement of the upper table 5U, as in a moving speedpattern shown in FIG. 7 in which a vertical axis is indicated by aninstructed moving speed VO of the ram and a horizontal axis is indicatedby a time T. Thereafter, commands are given to stop an increase of themoving speed, move at a fixed speed only for a predetermined warmup timeTW and thereafter increase the moving speed again. Further, a commandposition counter 67 reads the position of the upper table 5U on thebasis of the moving speed pattern given from the ram moving speedpattern command portion 65.

A position counter 71 feeds back a position signal 69 given from thelinear scale 11 that detects the position of the upper table 5U. Anadder 73 adds the feed-back position signal and a command position readby the command position counter 67 mentioned above. A ram motion gaindetermining portion 75 determines a gain on the basis of a signal addedby the adder 73, and a command is generated to the AC serve motor 39after being amplified by an amplifier 77.

According to the structure mentioned above, in the case that the workingfluid is charged into the upper cylinder chamber 19U and the lowercylinder chamber 19L, the bidirectional piston pump 31 stops and thepiston 19R rapidly moves the upper table 5U downward from a state ofbeing at a top dead center due to its own weight and the hydrauliccylinder 13R. The piping 41 and the piping 47 are connected by switchingthe sequence switch valve 45, and the bidirectional piston pump 31 isrotated by the AC-servo motor 39.

In the case of moving further downward to execute the bending process,the sequence switch valve is set to a state shown in FIG. 6. The workingfluid from the lower cylinder chamber 19L is returned to bidirectionalpiston pump 31 through the piping 41, the counter balance valve 43 andthe piping 47, and is supplied from the piping 29 to the upper cylinderchamber 19U in the hydraulic cylinder 13R. Accordingly, the piston 19Rmoves downward and the upper table 5U moves downward, thereby executingthe bending process.

Incidentally, since a cross sectional area in a lower surface side ofthe piston 19R is smaller than an upper surface side, an amount of theworking fluid returning to the bidirectional piston pump 31 from thelower cylinder chamber 19L is less than an amount of the working fluidcharged into the upper cylinder chamber 19U, so that the working fluidis refilled from the oil tank 27 via the check valve 51.

In the case that the working fluids in the upper and lower cylinderchambers 19U and 19L become high pressure, the structure is made suchthat a part of the working fluid is returned to the oil tank 27 from therelief valve 61 via the high pressure preference type shuttle valve 57through a piping 63.

In the case of reversing the hydraulic cylinder 13R on the basis of thepattern signal given from the ram moving speed pattern command portion65, so as to move the upper table 5U upward, the AC servo motor 39 isreverse rotated in an opposite direction (to that of the case mentionedabove) on the basis of the reverse rotation command, so as to reverserotate the bidirectional piston pump 31. The working fluid from theupper cylinder chamber 19U, in a state in which the piston 19R movesdownward, is supplied to the lower cylinder chamber 19L through thepiping 29, the bidirectional piston pump 31, the piping 47, the switchvalve 45, the piping 41 and the like. Accordingly, the piston 19R movesupward and the upper table 5U starts moving upward.

Further, when the command position counter 67 reads the ram moving speedpattern given from the ram moving speed pattern command portion 65, andthe piston 19R reaches a predetermined upward moving speed, a command isgiven so that acceleration is stopped so as to move upward at a fixedspeed for a predetermined warmup time TW. The check valve 51 is securelyclosed during this period. Thereafter, when the warmup time TW haspassed, the check valve 51 is closed and there is generated a state inwhich a back flow of the working fluid does not occur, an accelerationis executed until an upward moving speed of the upper table 5U reaches apredetermined speed. Acceleration is executed by controlling the ACservo motor 39.

Incidentally, when a pressure of the working fluid charged into thelower cylinder chamber 19L becomes higher than a predetermined value,the prefill valve 23 is opened according to a pilot signal 79, and theworking fluid is fed to the oil tank 27 from the upper cylinder chamber19U through the prefill valve 23.

As a result of the above, the structure is made such that the warmuptime TW is provided to temporarily keep the moving speed fixed in thecourse of the low moving speed of the upper table 5U, after reverserotating the bidirectional piston pump 31. The check valves 35 and 51are closed before the great positive pressure is applied. Accordingly,as shown in FIG. 8, in which a vertical axis is indicated by an actualspeed VR of the ram and a horizontal axis is indicated by a time T, itis possible to reduce the shock at the rising time due to the surgepressure which is conventionally a problem (refer to FIG. 3), and it ispossible to prevent the upper table 5U from being vibrated at a time ofmoving. Therefore, it is possible to increase a motion gain of the uppertable 5U so as to improve a productivity.

Of course, the present invention can be carried out according to theother aspects by executing a suitable modification without being limitedto the embodiment mentioned above. That is, in the embodiment mentionedabove, the press brake 1 moves the upper table 5U upward and downward ashas been explained; however, the benefits of the present invention canbe obtained when a press brake moves the lower table 5L upward anddownward.

Further, the warmup for keeping the ram speed fixed may be executed fora fixed ram moving distance.

A second embodiment will be explained below with reference to thedrawings.

Since the bidirectional fluid pump described in the first embodiment isused under a high rotation and a high pressure, there is an advantagethat it is possible to make a capacity of the servo motor driving thebidirectional fluid pump small.

However, the bidirectional fluid pump mentioned above generates a noisewhen being used at a high rotation. Further, when being used at a highrotation and a high pressure, it has a nature of generating furthergreat noise.

Accordingly, as shown in FIG. 9, in the case of vertically moving theram according to the ram moving pattern (a solid line in FIG. 9), so asto execute the bending process, an actual ram moving speed VR (shown bya broken line in FIG. 9) is reduced so as to be deviated from the ramspeed command value VO at a time T1 when the punch is brought intocontact with a work or during the bending process. Accordingly, in orderto remove the deviation and move the actual speed close to the commandspeed, an amount of rotation R of the servo motor is increased so as tomake the rotation of the bidirectional fluid pump high as shown in FIG.10. Accompanying this, as shown in FIG. 11, there is a problem in thatthe noise becomes great.

Further, as shown by a two-dot chain line in FIG. 9, since thebidirectional fluid pump is used under the high pressure P at a time T1when the punch is brought into contact with the work and during thelater bending process, there is a problem that a further great noise isgenerated.

The press brake according to the second embodiment corresponds to animprovement of the press brake according to the first embodiment.

Since a main body portion of the press brake according to the secondembodiment of this invention is the same as the main body portion of thepress brake 1 according to the first embodiment, explanation thereofwill be omitted.

A control apparatus 219 with respect to the hydraulic cylinders 13L and13R will be explained with reference to FIG. 12. Incidentally, since thesame control is applied to the left and right hydraulic cylinders 13Land 13R, a control of an AC servo motor 223 that corresponds to a servomotor that rotates a bidirectional piston pump 221 (corresponding to abidirectional fluid pump for the right hydraulic cylinder 13R) will beexplained as follows.

That is, in this control apparatus 219, there is provided a ram movingspeed pattern command portion 225 that instructs a moving speed patternfor moving, for example, the upper table 5U corresponding to the ram. Inthis ram moving speed pattern command portion 225, an upward anddownward movement of the upper table 5U is set according to a movingspeed pattern shown in FIG. 12. Further, a command position counter 227reads a command position of the upper table 5U on the basis of a commandpattern given from the ram moving speed pattern command portion 225.

A position counter 229 reads an actual position signal given from thelinear scale 11 (the ram position detecting means) that detects theposition of the upper table 5U. The actual position signal is read so asto be fed back. An adder 231 adds the feed-back signal and the commandposition read by the command position counter 227 mentioned above so asto compare. A ram motion gain determining portion 233 determines a rammotion gain on the basis of a signal added by the adder 231. A servomotor rotational amount command portion 235 is connected to the rammotion gain determining portion 233. A signal given from the servo motorrotational amount command portion 235 is amplified by an amplifier 237and a command is output to the AC servo motor 223.

Incidentally, a pressure sensor 239 provided in the bidirectional pistonpump 221, a computing portion 241 computing a change amount of pressureon the basis of a pressure given from the pressure sensor 239, and amemory 243 storing a relation between a pressure and a ram moving speedand a relation between a change amount of pressure and a ram movingspeed, are connected to a ram speed cramp value determining portion 295that determines a moving speed of the upper table 5U corresponding tothe ram. This ram speed cramp value determining portion 245 is connectedto a servo motor rotational amount command portion 235 that instructs arotational amount of the AC servo motor 223 that corresponds to the rammoving speed determined by the ram motion gain determining portion 233.

In FIG. 13, there is shown an absolute amount PQ (shown by a solid linein FIG. 13) of the pressure of the bidirectional piston pump 221 and achange amount PV (shown by a single-dot chain line in FIG. 13) of thepressure in the case of executing the bending process. The absoluteamount PQ of the pressure starts increasing at a time T1, when the punchP is brought into contact with the work, and the absolute amount PQ ofthe pressure gradually increases during the bending process.

Accordingly, a first derivative corresponding to the change amount PV ofthe pressure rapidly rises from the time T1 when the punch P is broughtinto contact with the work, and becomes substantially fixed during theperiod when the bending process is executed at a fixed pressure.Further, when the absolute amount PQ of the pressure becomes fixed, thechange amount PV of the pressure becomes zero.

Further, in FIG. 14, there is shown a ram moving speed VR which ispreviously stored in the memory 243, taking the noise of thebidirectional piston pump 221 into consideration, and which should beset with respect to the change amount PV of the pressure. Further, inFIG. 15, there is shown a ram moving speed VR which is previously storedin the memory 243, taking the noise of the bidirectional piston pump 221into consideration, and which should be set with respect to the absoluteamount PQ of the pressure.

As mentioned above, since the noise is increased at a time when thebidirectional piston pump 221 is under the high rotation and the highpressure, a value A1 of the change amount PV of the pressure and a valueA2 of the absolute amount PQ of the pressure in a time T1 are calculatedin the graph shown in FIG. 13. Ram moving speeds B1 and B2, which areram moving speeds to be set with respect to the pressure and the changeamount of pressure, are respectively calculated on the basis of FIGS. 14and 15. As a result of comparing the ram moving speeds B1 and B2, andsetting the lower speed as the ram speed clamp value, in the case thatthe speed computed by the ram motion gain determining portion 233 islarger than the ram speed clamp value, the ram speed clamp value isinstructed to the AC servo motor 223.

Accordingly, in the embodiment shown in FIGS. 13, 14 and 15, thestructure is made such that the ram moving speed B1 is employed, and therotational amount, corresponding to the smaller value between the rammoving speed B1 and the value computed by the ram motion gaindetermining portion 233, is provided to the AC servo motor 223 as aninstruction.

According to the structure mentioned above, the command position counter227 reads the command position of the upper table 5U according to thepattern given from the ram moving speed pattern command portion 225.This position and the actual position, read by the position counter 229on the basis of the position signal of the linear scale 11, are comparedby the adder 231, and the ram motion gain determining portion 233determines the gain. Here, the servo motor rotational amount commandportion 235 compares the rotational amount corresponding to the ramspeed determined by the ram speed clamp value determining portion 245with the rotational amount computed by the ram motion gain determiningportion 233, determines the smaller rotational amount and provides thesmaller rotational amount to the AC servo motor 223 as an instruction,and rotates the bidirectional piston pump 221.

According to the results mentioned above, since it is possible torestrict the rotational amount, at a time of high speed rotation andhigh pressure rotation of the bidirectional piston pump 221, to aminimum rotational amount, it is possible to restrict the generation ofnoise to be equal to or less than a fixed level.

Incidentally, in the same manner as the first embodiment, this inventioncan be carried out according to other aspects by executing a suitablemodification without being limited by the above-described embodiment ofthe invention. That is, in the embodiment of the invention mentionedabove, the press brake 1 moves the upper table 5U upward and downward asthe ram so as to execute the bending process; however, the same aspectscan be applied to moving the lower table 5L upward and downward so as toexecute the bending process.

1. A press brake, comprising: a hydraulic cylinder that vertically movesa movable ram; a bidirectional fluid pump that is bidirectionallyrotated by a servo motor and that operates the hydraulic cylinder tovertically move the ram; and a controller that controls the servo motor,the controller comprising: a ram moving speed pattern setter that sets aram moving speed pattern to maintain a ram speed for one of apredetermined time and a predetermined distance after a rotation of thebidirectional fluid pump reversed, and that thereafter changes the ramspeed to a predetermined speed; a counter that reads a ram positionbased on the ram speed instructed by the ram moving speed patternsetter; a position detector that detects the actual ram position; and anadder that compares the position read by the counter and the actualposition detected by the position detector to conform the operation ofthe servo motor to the ram moving speed pattern.
 2. A method forcontrolling a bidirectional fluid pump of a press brake, the methodcomprising: reversing the bidirectional fluid pump to reverse a verticalmovement of a ram that is moved by a hydraulic cylinder; setting one ofa predetermined time and a predetermined distance to maintain a ramspeed after the bidirectional fluid pump is reversed; and controllingthe bidirectional fluid pump to change the ram speed to a predeterminedspeed after the ram is moved for one of the predetermined time and thepredetermined distance.
 3. A method for controlling a bidirectionalfluid pump of a press brake, the method comprising: measuring ahydraulic force in a bidirectional fluid pump and computing a change inthe hydraulic force; calculating a ram speed based on at least one of apredetermined relationship between the speed and a detected pressure andbetween the speed and a detected change in the pressure; and instructinga servo motor to rotate the bidirectional fluid pump, by an amountcorresponding to the calculated speed, to vertically move the ram at thecalculated speed.
 4. A method for controlling a bidirectional fluid pumpof a press brake, the method comprising: measuring a hydraulic force ina bidirectional fluid pump and computing a change in the hydraulicforce; calculating a ram speed based on a predetermined relationshipbetween the speed and a detected pressure, and calculating a ram speedbased on a predetermined relationship between the speed and a detectedchange in the pressure; obtaining the lower of the calculated speeds;and instructing the servo motor to rotate the bidirectional fluid pump,by an amount corresponding to the lower of the calculated speeds, tovertically move the ram at the lower speed.
 5. A press brake,comprising: a hydraulic cylinder that vertically moves a movable ram; abidirectional fluid pump that is rotated bidirectionally by a servomotor and that operates the hydraulic cylinder to vertically move theram; a moving speed pattern setter that sets a ram moving speed pattern;a pressure sensor that determines one of a pressure and a pressurechange; a speed calculator that calculates a ram speed, based on one ofthe pressure and the pressure change; and a rotation amount instructorthat instructs a servo motor to rotate the bidirectional fluid pump, byan amount corresponding to the calculated speed, to move the ram at thecalculated speed.
 6. A press brake, comprising: a hydraulic cylinderthat vertically moves a movable ram; a bidirectional fluid pump that isrotated bidirectionally by a servo motor and that operates the hydrauliccylinder to move the ram; a position detector that detects a position ofthe ram; a moving speed pattern setter that sets a moving pattern of theram; an adder that compares a set ram position from the moving speedpattern setter with the detected ram position to conform the operationof the servo motor to the ram moving speed pattern; a pressure computerthat computes a pressure change of the bidirectional fluid pump, basedon a pressure detected by a pressure sensor; a memory that storesrelationships between the speed and the detected pressure and betweenthe speed and the pressure change; and a servo motor rotation setterthat determines the lower of a speed calculated based on a relationshipbetween the speed and the detected pressure and a speed calculated basedon a relationship between the speed and the pressure change, and thatinstructs the servo motor to rotate the bidirectional fluid pump, by anamount corresponding to the determined speed, to move the ram at thelower speed.